Most wells of the type used in producing fluids (e.g. hydrocarbons) from subterranean formations are completed by cementing a string of well casing within the wellbore. The casing is then perforated adjacent the formation(s) of interest so that fluids can be injected/produced therethrough. As is well known in the art, there may be times when it becomes necessary to remove a section of this casing (i.e. cut or mill a "window" through the casing and cement) in order to carry out a required or desired operation.
For example, it is common to mill a window(s) in a well tubular (e.g. well casing) in order to drill a curved or inclined "drain hole(s)" outward from the cased wellbore into a formation of interest. The drain hole, which aids in producing fluids from the formation, may consist of a horizontal wellbore which is drilled outward from a substantially vertical, cased wellbore or, as will be understood in this art, it may be a wellbore which extends upward/downward or otherwise radially outward from a substantially horizontal or inclined, cased wellbore. In addition to the drilling of drain holes, there are also several other operations which might require the "cutting of a window(s)" in a well casing, e.g. sidetracking, all of which are well known to those skilled in the art.
There are several types of tools which have been developed for milling windows in well tubulars. For example, where a conventional drilling rig is used, a deflecting tool, commonly called a "whipstock" is lowered into the well casing and is supported at the desired depth by a concrete plug, an expanding anchor, or the like. The whipstock is properly oriented to insure that the window will be milled in the appropriate direction through the casing. A starting mill on the lower end of a rotating drill string is then lowered into engagement with the whipstock which, in turn, deflects the mill into contact with the casing to thereby mill a relatively short pilot hole in the casing. The drill string is retracted and the starting mill is replaced with a window mill or other speciality mill to complete the window-cutting operation. For a further description of this type of window-cutting operation, see U.S. Pat. No. 4,397,360, issued Aug. 9, 1983 and also, the prior art processes disclosed and fully discussed in both U.S. Pat. Nos. 5,277,251 and 5,287,921,
Due to the expense normally associated with conventional drilling rigs, there has recently been a trend towards using commercially-available "coiled tubing units" for the milling of windows and other related operations whenever and wherever practical; see U.S. Pat. Nos. 5,277,251 and 5,287,921. As will be understood in the art, a typical coiled tubing unit is basically comprised of a continuous length of tubing which is wound on a large diameter drum and which can be fed into and out of a wellbore without having to "make up" or "break out" individual joints of the tubing.
If rotation is needed for a particular operation, a downhole motor is connected onto the lower end of the coiled tubing string to drive the mill. Further, since most coiled tubing strings and related tools are designed to be run and operated through a string of well tubing (e.g. such as the production tubing normally present in a cased well), there is no need to remove and replace the tubing from a well when carrying out a desired operation with a coiled tubing unit. This feature alone can amount to substantial savings in both time and expense in most operations.
In cutting a window in the well casing, either with conventional drilling rigs or with coiled tubing units, it is important that the cutting or milling surfaces of the mill be directed into contact with the casing without any substantial contact with the whipstock, itself. That is, the rotation of the mill has a tendency to draw the cutting surface of the mill into the first surface it contacts. It follows that if the mill contacts the casing first, the continued rotation of the mill will cause the mill to pull itself into the casing thereby cutting the desired window therein. However, if the cutting surfaces of the mill contacts the inclined or tapered surface of the whipstock first, then continued rotation will cause the mill to "dig in" into and cut the whipstock instead of the casing, thereby adversely affecting the window cutting operation.
In typical prior art operations, a tapered, non-cutting surface on the pilot or starting mill cooperates with a wear pad or the like on the whipstock to direct the cutting surfaces of pilot mill into contact with the casing while preventing the cutting surfaces from contacting the whipstock. While successful, only a very short pilot hole is normally cut into the casing before the non-cutting surface of the starting mill contacts the casing thereby interferring with any further cutting of the window. This requires "tripping" the drill string to replace the starting mill with a window mill or the like in order to provide a window having an adequate length to allow the subsequent operation to be carried out. Accordingly, a need exists for improved whipstocks and mills which can initiate and mill a window of substantial length with a miminum of manipulated steps by insuring that the cutting surfaces of the mill will contact the casing rather than the tapered surface of the whipstock.